Method of producing diamines and polyamines of the diphenylmethane series at different production capacities

ABSTRACT

The invention relates to a method for producing diamines and polyamines of the diphenylmethane series, by condensing aniline and formaldehyde followed by an acid-catalysed rearrangement at different production capacities. Adapting the respective molar ratios of the total used aniline to the total used formaldehyde and of the total used acid catalyst to the total used aniline allows the rearrangement reaction to be fully completed despite the change in dwell time inevitably associated with a change in production capacity and ensures that no undesired fundamental changes occur in the product composition.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage application under 35 U.S.C. § 371of PCT/EP2017/083260, filed Dec. 18, 2017, which claims the benefit ofEuropean Application No. 16206113.9, filed Dec. 22, 2016, both of whichare incorporated by reference herein.

FIELD

The present invention relates to a process for preparing di- andpolyamines of the diphenylmethane series by condensation of aniline andformaldehyde followed by acid-catalyzed rearrangement at differentproduction capacities. Adjusting the respective molar ratios of totalaniline used to total formaldehyde used and of total acidic catalystused to total aniline used achieves running of the rearrangementreaction to completion in spite of the change in residence timeinevitably associated with the altered production capacity and avoidingunwanted significant changes in the product composition.

BACKGROUND

The preparation of di- and polyamines of the diphenylmethane series(MDA) by reaction of aniline with formaldehyde in the presence of acidiccatalysts is known in principle. In the context of the presentinvention, di- and polyamines of the diphenylmethane series areunderstood to mean amines and mixtures of amines of the following type:

n here is a natural number≥2. The compounds of this type in which n=2are referred to hereinafter as diamines of the diphenylmethane series ordiaminodiphenylmethanes (MMDA hereinafter; “monomer MDA”). Compounds ofthis type in which n>2 are referred to in the context of this inventionas polyamines of the diphenylmethane series or polyphenylenepolymethylene polyamines (PMDA hereinafter; “polymer MDA”). Mixtures ofthe two types are referred to as di- and polyamines of thediphenylmethane series (for the sake of simplicity referred tohereinafter as MDA). The corresponding isocyanates that can be derivedin a formal sense by replacing all NH₂ groups with NCO groups from thecompounds of the formula (I) are accordingly referred to asdiisocyanates of the diphenylmethane series (MMDI hereinafter),polyisocyanates of the diphenylmethane series or polyphenylenepolymethylene polyisocyanates (PMDI hereinafter) or di- andpolyisocyanates of the diphenylmethane series (MDI hereinafter). Thehigher homologs (n>2), both in the case of the amine and in the case ofthe isocyanate, are generally always present in a mixture with thedimers (n=2), and so only two product types are of relevance inpractice, the pure dimers (MMDA/MMDI) and the mixture of dimers andhigher homologs (MDA/MDI). The position of the amino groups on the twophenylene groups in the dimers (para-para; ortho-para and ortho-ortho)is specified hereinafter only when it is important. For the sake ofsimplicity, this is done in the X,Y′-MDA form (4,4′-, 2,4′- or2,2′-MDA), as is customary in the literature. The same is true of MDI(identification of the isomers as X,Y′-MDI (4,4′-, 2,4′- or 2,2′-MDI).

Industrially, the di- and polyamine mixtures are converted predominantlyby phosgenation to the corresponding di- and polyisocyanates of thediphenylmethane series.

The continuous or partly discontinuous preparation of MDA is disclosed,for example, in EP-A-1 616 890, U.S. Pat. No. 5,286,760, EP-A-0 451 442and WO-A-99/40059. The acidic condensation of aromatic amines andformaldehyde to give di- and polyamines of the diphenylmethane seriesproceeds in multiple reaction steps. In what is called the “aminalprocess”, in the absence of an acidic catalyst, formaldehyde is firstcondensed with aniline to give what is called aminal, with eliminationof water. This is followed by the acid-catalyzed rearrangement to MDA ina first step to give para- or ortho-aminobenzylaniline. Theaminobenzylanilines are converted to MDA in a second step. Main productsof the acid-catalyzed reaction of aniline and formaldehyde are thediamine 4,4′-MDA, its positional isomers 2,4′-MDA and 2,2′-MDA and thehigher homologs (PMDA) of the various diamines. In what is called the“neutralization process”, aniline and formaldehyde are converted in thepresence of an acidic catalyst directly to aminobenzylanilines, whichare then rearranged further to give the bicyclic MMDA isomers and higherpolycyclic PMDA homologs. The present invention relates to the aminalprocess.

Irrespective of the process variant for preparation of the acidicreaction mixture, the workup thereof, according to the prior art, isinitiated by neutralization with a base. This neutralization istypically effected at temperatures of, for example, 90° C. to 100° C.without addition of further substances (cf. H. J. Twitchett, Chem. Soc.Rev. 3(2), 223 (1974)). It can alternatively be effected at a differenttemperature level in order, for example, to accelerate the degradationof troublesome by-products. Hydroxides of the alkali metal and alkalineearth metal elements are suitable as bases. Preference is given to usingsodium hydroxide solution.

After the neutralization, the organic phase is separated from theaqueous phase in a separating vessel. The organic phase which comprisescrude MDA and remains after removal of the aqueous phase is subjected tofurther workup steps, for example washing with water (base washing) inorder to wash residual salts out of the crude MDA. Finally, the crudeMDA thus purified is freed of excess aniline, water and other substancespresent in the mixture (e.g. solvents) by suitable methods, for exampledistillation, extraction or crystallization. The workup which iscustomary according to the prior art is disclosed, for example, inEP-A-1 652 835, page 3 line 58 to page 4 line 13, or EP-A-2 103 595,page 5 lines 21 to 37.

International patent application WO 2014/173856 A1 provides a processfor preparing di- and polyamines of the diphenylmethane series byconverting aniline and formaldehyde in the absence of an acidic catalystto aminal and water, removing the aqueous phase and processing theorganic aminal phase further to give the di- and polyamines of thediphenylmethane series, in which use of a coalescence aid in the phaseseparation of the process product obtained in the aminal reactionreduces the proportion of water and hence also of water-solubleimpurities in the organic phase comprising the aminal. The di- andpolyamines of the diphenylmethane series that are obtained byacid-catalyzed rearrangement and workup after further processing of theaminal phase are of excellent suitability for preparation of thecorresponding isocyanates.

The quality of a reaction process for preparation of MDA is definedfirstly by the content of unwanted secondary components and impuritiesin the crude product that can arise from improper conduct of thereaction. Secondly, the quality of a reaction process is defined in thatthe entire process can be operated without technical production outageor problems that necessitate intervention in the process, and thatlosses of feedstocks are prevented or at least minimized.

Although the prior art processes described succeed in preparing MDA witha high yield and without loss of quality in the end products, the onlyprocesses described are in the normal state of operation. Only a fewpublications are concerned with states outside normal operation:

International patent application WO 2015/197527 A1 relates to a processfor preparing di- and polyamines of the diphenylmethane series (MDA), toa plant for preparation of MDA and to a method of operating a plant forpreparation of MDA. The invention enables optimization of productionshutdowns in the operation of the MDA process with regard to time takenand optionally also with regard to energy and material consumption bymeans of what is called a circulation mode in individual plantcomponents. During an interruption in the process or interruption of theoperation of individual plant components, there is no introduction offormaldehyde into the reaction, and the plant components that are notaffected by an inspection, repair or cleaning measure are operated inwhat is called circulation mode. What this achieves, among othereffects, is that only the plant component in question must be shut downfor the period of the measure, which may be advantageous in terms ofproductivity and economic viability of the process and the quality ofthe products prepared.

International patent application WO 2015/197519 A1 relates to a processfor preparing diamines and polyamines of the diphenylmethane series, inwhich care is taken during the running-down of the production processthat an excess of aniline over formalin is ensured.

International patent application WO 2015/197520 A1 relates to a processfor preparing diamines and polyamines of the diphenylmethane series(MDA) from aniline and formaldehyde, in which care is taken during thestart-up procedure to ensure that there is a sufficient excess ofaniline over formaldehyde which is at least 105% of the molar ratio ofaniline to formaldehyde wanted for the target formulation of the MDA tobe produced.

Changes in the target production capacity (also called “change in load”)during continuous production (i.e. with a starting state and an endstate, in which MDA is produced) are not taken into account here either.Since aniline is typically used in stoichiometric excess, the productioncapacity of a given plant for preparation of MDA is defined by theformaldehyde feed which, in the context of this invention, is referredto as (formaldehyde) load.

Changes in load are recurrent plant states that have a considerableeffect on the economic (and environmentally benign in terms of energyconsumption) operation of a continuously operating plant. Since, for agiven continuously operated production plant, a change in load isinevitably associated with a change in the residence time of thereaction mixture in the reaction space available, as well as purelyeconomic, environmental and operational challenges, there mayadditionally be an unwanted change in the composition of the product.This is because the exact composition of the MDA obtained (especiallythe isomer ratio of MMDA and the ratio of MMDA to PMDA) is alsocrucially dependent on the residence time of the reaction mixture, andso, in the event of improper procedure, a significantly differentproduct can be obtained after a change in load, even though this was notintended.

It would thus be desirable to have available a process for preparing di-and polyamines of the diphenylmethane series in which it is possible bysimple measures to configure changes in load in the operation of the MDAprocess such that they run in an optimized manner (for instance withregard to yield, time taken, energy consumption and avoidance ofprocess-related problems such as caking or blockages in apparatus) ineconomic, environmental and operational aspects with avoidance ofunwanted changes in the product composition.

SUMMARY

According to the invention, this can be accomplished by a processaccording to patent claim 1. This process, as will be elucidated indetail hereinafter, is more particularly characterized in that in theevent of an intended increase in load the molar ratio of total anilineused to total formaldehyde used, n(1)/n(2) (in the literature alsoreferred to as A/F ratio), is lowered and the molar ratio of totalacidic catalyst used to total aniline used, n(7)/n(1) (in the literaturealso as protonation level=n(7)/n(1)·100%), is increased and in that inthe event of a decrease in load the reverse procedure is followed.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and characteristics of the inventions described in thisspecification may be better understood by reference to the accompanyingfigures, in which:

FIGS. 1a-1c illustrate the procedure of the invention and show the ration(1)/n(2) and the flow rate m₂, each as a function of time t;

FIGS. 2a-b illustrate the procedure of the invention and show the ration(7)/n(1) and the flow rate m₂, each as a function of time t;

FIG. 3 illustrates a production plant suitable for the performance ofthe process of the invention; and

FIG. 4 shows a detail of the reactor cascade.

DETAILED DESCRIPTION

All configurations of the present invention relate to a process forpreparing di- and polyamines of the diphenylmethane series from aniline(1) and formaldehyde (2) in a production plant (10 000), where the molarratio of total aniline used (1) to total formaldehyde used (2),n(1)/n(2), is always greater than 1.6, comprising the steps of:

(A-I) reacting aniline (1) and formaldehyde (2) in the absence of anacidic catalyst in a reactor (1000, aminal reactor) to obtain a reactionmixture (4) comprising an aminal (3), and then at least partlyseparating an aqueous phase (6) from the reaction mixture (4) in aremoval unit (2000, also called aminal separator) to obtain an organicphase (5) comprising the aminal (3);

(A-II) contacting the organic phase (5) which comprises the aminal andis obtained in step (A-I) with an acidic catalyst (7) in a reactorcascade (3000) composed of i reactors j connected in series (3000-j=1,3000-j=2, . . . , 3000-j=i; hereinafter 3000-1, 3000-2, . . . , 3000-ifor short), where i is a natural number from 2 to 10 (acid-catalyzedrearrangement), wherein

-   -   the first reactor (3000-1) in flow direction is operated at a        temperature T₃₀₀₀₋₁ in the range from 25.0° C. to 65.0° C.,        preferably in the range from 30.0° C. to 60.0° C., and is        charged with stream (5) and acidic catalyst (7) and optionally        with further aniline (1) and/or further formaldehyde (2),    -   every reactor downstream in flow direction (3000-2, . . . ,        3000-i) is operated at a temperature of more than 2.0° C. above        T₃₀₀₀₋₁ and is charged with the reaction mixture obtained in the        reactor immediately upstream;

(B) isolating the di- and polyamines of the diphenylmethane series fromthe reaction mixture (8-i) obtained from step (A-II) in the last reactor(3000-i) by at least

(B-I) adding a stoichiometric excess of base (9), based on the totalamount of acidic catalyst used (7), to the reaction mixture (8-i)obtained in the last reactor (3000-i) in step (A-II) in a neutralizationunit (4000, preferably stirred neutralization vessel) to obtain areaction mixture (10), especially observing a molar ratio of base (9) tototal acidic catalyst used (7) in the range from 1.01 to 1.30(neutralization);

(B-II) separating the reaction mixture (10) obtained in step (B-I) in aseparation unit (5000, neutralization separator) into an organic phase(11) comprising di- and polyamines of the diphenylmethane series and anaqueous phase (12).

The expressions “total aniline used” and “total formaldehyde used” eachrefer to the total amount of these reactants used in the processaccording to the invention, i.e. including any proportions additionallyadded after step (A-I).

The organic phase (11) can, as described in detail hereinafter, besubjected to further workup.

According to the invention, the abovementioned steps are conductedcontinuously, meaning that the respective reactants are suppliedcontinuously to the apparatus assigned to the respective step and theproducts are withdrawn continuously therefrom.

According to the invention—in simplified terms; details are elucidatedfurther down—the procedure is thus that, in the event of a change inload, the temperature in the reactors 3000-2, . . . , 3000-i is keptessentially constant and the molar ratio of total acidic catalyst usedto total aniline used (n(7)/n(1) hereinafter) and the molar ratio oftotal aniline used to total formaldehyde used (n(1)/n(2) hereinafter) isadjusted such that, in spite of altered residence time, therearrangement reaction runs to completion and unwanted changes in theproduct composition and increased by-product formation as a consequenceof the change in load are largely avoided. According to the invention,the temperature in each reactor j of the reactor cascade (3000) afterthe change in load is essentially the same as before the change in load,although, in the case of the first reactor (3000-1) in flow direction,slightly greater deviations from the temperature before the change inload can be permitted during the change in load. These slightly greatertolerances in the temperature during the change in load allow easiercontrol over the change in load in terms of control technology. Thetolerances permitted in accordance with the invention, which are stillto be elucidated in detail further down, however, are still within sucha scope that operational problems are largely to completely avoided.

This procedure allows advantageous configuration of changes in load ineconomic, environmental (from the point of view of energy consumption)and operational aspects.

Since the invention relates to changes in load, i.e. to different statesof operation of a production plant, the following definitions of termsthat are used hereinafter are helpful:

Starting state A of the production plant with a given productioncapacity defined by the amount of formaldehyde introduced:

-   -   mass flow rate m₁ of total aniline used in the starting state:        m₁(A)≠0 [e.g. in t/h],    -   mass flow rate m₂ of total formaldehyde used in the starting        state: m₂(A)≠0 [e.g. in t/h],    -   mass flow rate m₇ of total acidic catalyst used in the starting        state: m₇(A)≠0 [e.g. in t/h]    -   molar ratio n(1)/n(2) of total aniline used (1) to total        formaldehyde used (2) in the starting state: n(1)/n(2)(A) and    -   molar ratio n(7)/n(1) of total acidic catalyst used to total        aniline used in the starting state: n(7)/n(1)(A).

End state E of the production plant with a target production capacitydefined by the amount of formaldehyde introduced (with introduction ofless or more formaldehyde compared to the starting state):

-   -   mass flow rate m₁ of total aniline used in the end state:        m₁(E)≠0 [e.g. in t/h],    -   mass flow rate m₂ of total formaldehyde used in the end state:        m₂(E)≠0 [e.g. in t/h],    -   mass flow rate m₇ of total acidic catalyst used in the end        state: m₇(E)≠0 [e.g. in t/h],    -   molar ratio n(1)/n(2) of total aniline used (1) to total        formaldehyde used (2) in the end state: n(1)/n(2)(E) and    -   molar ratio n(7)/n(1) of total acidic catalyst used to total        aniline used in the end state: n(7)/n(1)(E).

Transition state T between starting state and end state (correspondingto the period of the change in load):

-   -   mass flow rate m₁ of total aniline used in the transition state:        m₁(T)≠0 [e.g. in t/h],    -   mass flow rate m₂ of total formaldehyde used in the transition        state: m₂(T)≠0 [e.g. in t/h],    -   mass flow rate m₇ of total acidic catalyst used in the end        state: m₇(E)≠0 [e.g. in t/h],    -   molar ratio n(1)/n(2) of total aniline used (1) to total        formaldehyde used (2) in the transition state: n(1)/n(2)(T),    -   molar ratio n(7)/n(1) of total acidic catalyst used (7) to total        aniline used in the transition state: n(7)/n(1)(T).

According to the invention, the transition state begins as soon as m₂ isincreased or lowered proceeding from m₂(A) (this time is referred tohereinafter as t₁), and it ends as soon as m₂ reaches the target valuefor m₂(E) (this time is referred to hereinafter as t₂).

On the basis of the minimum value for n(1)/n(2) of 1.6 used in theprocess of the invention, the yield of MDA target product is limited bythe flow rate of total formaldehyde used (2) which is introduced. Themaximum possible flow rate of total formaldehyde used (2) forpreparation of a particular product type (i.e. a mixture of di- andpolyamines of the diphenylmethane series characterized by a particularisomer composition of MMDA and a particular ratio of MMDA to PMDA) underthe given boundary conditions of the production plant (10 000):

-   -   m₂(N) [e.g. in t/h],        is referred to here and hereinafter as nameplate load.

The maximum possible flow rate of total formaldehyde used (2) may varyaccording to the MDA product type to be prepared. This is generallyunimportant for the present invention since the product after a changein load is essentially the same as before the change in load. If slightchanges in the product composition, contrary to expectation, should leadto significant deviation in the nameplate load in the end state fromthat in the starting state, it is the nameplate load in the startingstate m₂(N, A) that is crucial for the purposes of the present invention(i.e. more particularly for the purpose of quantification of the changein load; for details see below). The value of m₂(N, A) for a givenproduction plant is either known from the outset (for example becausethe plant was designed for the production of a particular amount of aparticular product type per hour) or can be easily determined by theperson skilled in the art from the known boundary conditions of theplant (number and size of apparatuses present and the like) for theknown operating parameters of the crucial starting state (n(1)/n(2)(A),n(7)/n(1)(A)). In practice, rather than m₂(N), the corresponding productflow rate of MDA (e.g. in t/h) which is to be expected under theassumption of the design parameters for the production plant (includingn(1)/n(2) ratio, n(7)/n(1) ratio etc.) is frequently reported. Ratherthan nameplate load, reference is therefore also frequently made tonameplate production capacity or else nameplate capacity for short. Inthis connection, the terms “load” and “(production) capacity” ultimatelymean the same thing, but have different reference points (flow rate ofstarting material in one case and flow rate of target product to beexpected from this starting material in the other case).

The actual load (or—see the above elucidations—the actual productioncapacity or capacity for short) in the starting state and end state,defined by the actual mass flow rate of total formaldehyde used in thestarting state or end state, can then be expressed in relation to thenameplate load as follows:

-   -   m₂(A)=X(A)·m₂(N) [e.g. in t/h],    -   m₂(E)=X(E)·m₂(N) [e.g. in t/h].

In both cases, X is a dimensionless multiplier greater than 0 and lessthan or equal to 1 that expresses the actual load in relation to thenameplate load. In the event of a change from production at half thenameplate load (“half-load”) to production with nameplate load (“fullload”), accordingly, X(A)=½ and X(E)=1. In the case of increases inload, X(E)>X(A); in the case of reductions in load, X(E)<X(A).

A quantitative measure for the size of a change in load is the magnitudeof the differential of X(E) and X(A):

-   -   |X(E)−X(A)|(=|X(A)−X(E)|)=ΔX

In all abovementioned states of operation, n(1)/n(2) is set to a valueof not less than 1.6. With regard to the expression “total acidiccatalyst used”, the statements made above for aniline and formaldehydeare correspondingly applicable: what is meant is the total amount ofacidic catalyst. This can be (and is preferably) fed entirely to thereactor 3000-1; alternatively, it is possible to supply this reactorwith just the majority of the total acidic catalyst used and to metersmaller proportions additionally into the reactors that follow in flowdirection.

According to the invention, moreover, the temperature in each of thereactors downstream in flow direction (3000-2, 3000-3, . . . 3000-i) isset to a value more than 2.0° C. above T₃₀₀₀₋₁. This is also true in allstates of operation, i.e. in the starting state, transition state andend state.

In any case, the aim is to largely avoid unwanted changes in the productcomposition resulting from the change in load.

According to the invention, the temperature of each reactor j of thereactor cascade (3000) in the end state is adjusted to a value thatequates to the respective temperature in the starting state within arange of variation of ±2.0° C.; in other words, in each case:

(T_(3000-j)(A)−2.0° C.)≤T_(3000-j)(E)≤(T_(3000-j)(A)+2.0° C.).

According to the invention, moreover, the temperature in the firstreactor in flow direction (3000-1) is always chosen such that it iswithin the range from 25.0° C. to 65.0° C., preferably 30.0° C. to 60.0°C. This is true in all states of operation, i.e. in the starting state,transition state and end state.

Moreover, during the transition state T:

(i) the temperature in the first reactor in flow direction (3000-1) fromstep (A-II) is set to a value that differs from the temperature in thisreactor during the starting state A by a maximum of 10.0° C., preferablyby a maximum of 5.0° C., more preferably by a maximum of 2.0° C., theparticularly preferred maximum deviation by ±2.0° C. being regarded asthe same temperature for all practical purposes (in the context of thepresent invention, slight nominal deviations from a temperature in theregion of up to ±2.0° C. are considered to be—essentially—the sametemperature; changes of greater than 2.0° C., by contrast, areconsidered to be significant temperature changes);

(ii) the temperature in all of the reactors downstream in flow direction(3000-2, . . . , 3000-i), by comparison with the starting state A, ineach case is kept the same within a range of variation of ±2.0° C.;

(iii-1) in the case that m₂(E)>m₂(A), n(1)/n(2)(T) and n(7)/n(1)(T) areadjusted such that, at the end of the transition state, i.e. when m₂ hasreached the target for m₂(E):

-   -   0.80·n(1)/n(2)(A)≤n(1)/n(2)(T)≤0.99·n(1)/n(2)(A), preferably    -   0.90·n(1)/n(2)(A)≤n(1)/n(2)(T)≤0.97 n(1)/n(2)(A);    -   and    -   1.01·n(7)/n(1)(A)≤n(7)/n(1)(T)≤2.00·n(7)/n(1)(A), preferably    -   1.05·n(7)/n(1)(A)≤n(7)/n(1)(T)≤1.50·n(7)/n(1)(A);        where it is always the case that, throughout the transition        state, prior to attainment of the time at which m₂ has reached        the target value for m₂(E):    -   0.80·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.20·n(1)/n(2)(A), preferably    -   0.90·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.10·n(1)/n(2)(A);    -   and    -   0.83·n(7)/n(1)(A)≤n(7)/n(1)(T)≤2.00·n(7)/n(1)(A), preferably    -   0.91·n(7)/n(1)(A)≤n(7)/n(1)(T)≤1.50·n(7)/n(1)(A);

(iii-2) in the case that m₂(E)<m₂(A), n(1)/n(2)(T) and n(7)/n(1)(T) areadjusted such that, at the end of the transition state, i.e. when m₂ hasreached the target value for m₂(E):

-   -   1.01·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.50·n(1)/n(2)(A), preferably    -   1.02·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.20·n(1)/n(2)(A);    -   and    -   0.50·n(7)/n(1)(A)≤n(7)/n(1)(T)≤0.99·n(7)/n(1)(A), preferably    -   0.60·n(7)/n(1)(A)≤n(7)/n(1)(T)≤0.80·n(7)/n(1)(A);        where it is always the case that, throughout the transition        state, prior to attainment of the time at which m₂ has reached        the target value for m₂(E):    -   0.90·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.50·n(1)/n(2)(A), preferably    -   0.95·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.20·n(1)/n(2)(A);    -   and    -   0.50·n(7)/n(1)(A)≤n(7)/n(1)(T)≤1.11·n(7)/n(1)(A), preferably    -   0.60·n(7)/n(1)(A)≤n(7)/n(1)(T)≤1.05·n(7)/n(1)(A).

According to the invention, for the reactor 3000-1 during the transitionstate, a significant deviation from the temperature in the startingstate is possible. In this reactor, changes in the amount of heatreleased as a consequence of the change in load are manifested to anenhanced degree. (Significant contributions to exothermicity are firstlythe heat of neutralization which is released in the reaction of the acidwith the aminic components of the reaction mixture, and theacid-catalyzed rearrangement of the aminal that sets in.) This can betaken into account if required by greater tolerances in the temperaturein the transition state. However, not later than at the end of thetransition state (i.e. when m₂ has reached the target value for m₂(E)),the temperature in the reactor 3000-1 is adjusted back to a value whichcorresponds to that of the starting state within a range of variation of±2.0° C.

The appended drawings are intended to illustrate the procedure of theinvention:

FIG. 1a-c show the ratio n(1)/n(2) and the flow rate m₂, each as afunction of time t.

FIG. 2a-b show the ratio n(7)/n(1) and the flow rate m₂, each as afunction of time t.

These and the other drawings are merely intended to illustrate the basicprinciple of the invention and do not claim to be true to scale.

It is essential to the invention that, at the end of the transitionstate (i.e. when the target load for the end state has been established,at t=t₂), in the case of an increase in load, the molar ratio of totalaniline used to total formaldehyde used (n(1)/n(2)) is lowered comparedto the starting state and the molar ratio of total acidic catalyst usedto total aniline used (n(7)/n(1)) is increased compared to the startingstate. In the case of a decrease in load, the procedure is reversed(increasing of n(1)/n(2) and lowering of n(7)/n(1)). The procedure ofthe invention is illustrated in FIG. 1a-c using the example of anincrease in load for the ratio n(1)/n(2): In the starting state, theload m₂=m₂(A). At time t=t₁ (=commencement of the transition state), theload is increased until, at time t=t₂ (=end of the transition state), ithas reached the target value for the end state m₂=m₂(E); in otherwords—and for all conceivable configurations of the invention and notjust cases shown by way of example in FIG. 1a-c —m₂(t=t₂)=m₂(E) andn(1)/n(2)(T)(t=t₂)=n(1)/n(2)(E). The lowering of n(1)/n(2) may, as shownin FIG. 1a , be continuous. However, this is not obligatory; forexample, it is also possible, as shown in FIG. 1b , first to increasen(1)/n(2) (by increasing m₁ to a greater degree at first than m₂) andonly to lower it to the target end value with a time delay, which leadsto an intermediate rise in n(1)/n(2). All that is essential is that thetarget end value for the ratio n(1)/n(2) is established during thetransition state in such a way that this target end valuen(1)/n(2)=n(1)/n(2)(E) exists at time t=t₂ (i.e. at the time at which m₂has reached the target value for m₂(E)). This is not restricted to thecase of an increase in load, as shown by way of example in FIG. 1a-c ,but is also true of all embodiments of the invention. The target endvalue n(1)/n(2)(E) can be established stepwise or, as shown in FIG. 1a-c, continuously, preference being given to continuous establishment. Thesame is true of the ratio n(7)/n(1)(FIG. 2a-c ). It is thereforelikewise the case—again for all conceivable configurations of theinvention and not just in the cases shown by way of example in FIG. 2a-c—that n(7)/n(1)(T)(t=t₂)=n(7)/n(1)(E). The respectively requiredadjustments in the operating parameter m₂ and at least one operatingparameter selected from m₁ and m₇ (preferably, both operating parametersm₁ and m₇ are altered) are preferably commenced at the same time andmore preferably conducted in such a way that the target values in eachcase for the end of the change in load (i.e. the time at which m₂ hasreached the target value for m₂(E)) are established via continuousadjustment of the respective operating parameter.

However, a slight time delay (generally in the order of magnitude ofseconds, preferably not more than 15 seconds) is possible in therequired adjustments of m₁, m₂ and m₇. This is especially effected insuch a way that, in the case of an increase in load, it commences firstwith the adjustment of m₁ (if such an adjustment, generally an increase,is required) and only then is m₂ increased with a slight time delay, insuch a way that the requirements of the invention on the ratio n(1)/n(2)are complied with. In the case of a decrease in load, the reverseprocedure is followed (commencement with the adjustment (i.e. in thiscase lowering) of m₂, and optionally adjustment, generally lowering, ofm₁, again of course with the proviso that the requirements of theinvention on the ratio n(1)/n(2) are complied with). In this embodimenttoo, the adjustment of m₇ required in each case is also undertakenpreferably at the same time as the adjustment of m₁. If, in thisembodiment, an increase in load is commenced with an increase in m₁before the increase in m₂ is commenced, there is already a rise in then(1)/n(2) ratio shortly before t=t₁ (i.e. shortly before commencement ofthe transition state as defined in accordance with the invention). It isgenerally the case that, when commencing with an adjustment of m₁ beforecommencement of the desired change (increase in load or decrease inload) of m₂ (such a time is shown in FIGS. 1c and 1s identified there byto), the ratios n(1)/n(2)(A) and n(7)/n(1)(A), for the purposes of thepresent invention, are calculated on the basis of the value of m₁ beforecommencement of the adjustment of m₁ (cf. FIG. 1c ).

Embodiments of the invention are described in detail hereinafter. Theseembodiments, unless stated otherwise in the specific case, areapplicable to all process regimes of the invention. It is possible hereto combine various embodiments with one another as desired, unless theopposite is apparent to the person skilled in the art from the context.

FIG. 3 shows a production plant (10 000) suitable for the performance ofthe process of the invention.

FIG. 4 shows a detail of the reactor cascade (3000). The merely optionaladdition of further acidic catalyst (7) to the reactors (3000-2) to(3000-i) downstream of the reactor (3000-1) is shown by dotted arrows.

Step (A-I) of the process of the invention, provided that the otherrequirements of the invention are complied with, can be conducted asknown in principle from the prior art. Aniline and aqueous formaldehydesolution are preferably condensed here at molar ratios in the range from1.6 to 20, preferably 1.6 to 10 and more preferably 1.6 to 6.0, evenmore preferably of 1.7 to 5.5 and very exceptionally preferably of 1.8to 5.0, at temperatures of 20.0° C. to 120.0° C., preferably 40.0° C. to110.0° C. and more preferably 60.0° C. to 100.0° C., to give aminal andwater. The reactor of step (A-I) is operated at standard pressure orunder elevated pressure. There is preferably a pressure of 1.05 bar to5.00 bar (absolute), very preferably of 1.10 bar to 3.00 bar (absolute)and most preferably of 1.20 bar to 2.00 bar (absolute). The pressure ismaintained by pressure-regulating valves, or by connecting the offgassystems of aminal reactor (1000) and the overflow from the aminalseparator (2000) used for phase separation on completion of reaction.The aminal separator and the outlet for the aqueous phase are preferablyheated in order to prevent caking.

Suitable aniline qualities are described, for example, in EP 1 257 522B1, EP 2 103 595 A1 and EP 1 813 598 B1. Preference is given to usingtechnical grade qualities of formalin (aqueous solution of formaldehyde)with 30.0% by mass to 50.0% by mass of formaldehyde in water. However,formaldehyde solutions with lower or higher concentrations or else theuse of gaseous formaldehyde are also conceivable.

The phase separation of organic aminal phase and aqueous phase ispreferably effected at temperatures of 20.0° C. to 120.0° C., morepreferably of 40.0° C. to 110.0° C. and most preferably of 60.0° C. to100.0° C., in each case preferably at ambient pressure or at slightlyelevated pressure relative to ambient pressure (elevated by up to 0.10bar) be carried out.

Step (A-II) of the process of the invention, provided that the otherrequirements of the invention are complied with, can be conducted asknown in principle from the prior art. The aminal is rearranged in thepresence of an acidic catalyst, typically a strong mineral acid such ashydrochloric acid. Preference is given to the use of mineral acid in amolar ratio of mineral acid to aniline of 0.0010 to 0.90, preferably0.050 to 0.50. It is of course also possible to use solid acidiccatalysts as described in the literature. Formaldehyde can be added hereto a mixture of aniline and acidic catalyst, and the reaction solutioncan be fully reacted by stepwise heating. Alternatively, aniline andformaldehyde can first be pre-reacted and then, with or without priorremoval of water, admixed with the acidic catalyst or a mixture offurther aniline and acidic catalyst, and then the reaction solution isfully reacted by stepwise heating. This reaction can be executedcontinuously or batch-wise by one of the numerous methods described inthe literature (for example in EP 1 616 890 A1 or EP 127 0544 A1).

It is possible to supply the first reactor 3000-1 with further anilineand/or further formaldehyde. It is likewise possible to supply thedownstream reactors 3000-2, . . . , 3000-i with small amounts of anilineand/or formaldehyde. These may each be fresh feedstocks or recyclestreams from other reactors. However, the majority of total aniline usedand of total formaldehyde used is introduced into the “aminal reactor”1000. Regardless of how aniline (1) and formaldehyde (2) aredistributed, optionally over various reactors, the process of theinvention is preferably conducted in such a way that the molar ratio oftotal aniline used (1) to total formaldehyde used (2), n(1)/n(2), in allstates of operation (A, T, E) always has a value of 1.6 to 2.0,preferably of 1.6 to 10, more preferably of 1.6 to 6.0, even morepreferably of 1.7 to 5.5 and very exceptionally preferably of 1.8 to5.0.

Preferably, all the acidic catalyst used (7) is fed completely toreactor 3000-1. Alternatively, it is possible to feed a portion of thetotal acidic catalyst used (7) to one or more of the reactors 3000-2, .. . , 3000-i downstream in flow direction.

The acidic catalyst (7) used in the process of the invention ispreferably a mineral acid, especially hydrochloric acid. Suitablehydrochloric acid qualities are described, for example, in EP 1 652 835A1.

Suitable reactors 3000-1, 3000-2, . . . 3000-i in step (A) in bothprocess regimes are apparatuses known to those skilled in the art, suchas stirred tanks and tubular reactors:

In the case of stirred tank reactors, the temperature is generally thesame throughout the reactor contents, and so, for the purposes of thepresent invention, it does not matter where the temperature is measured.If, contrary to expectation, significant temperature differences existthrough the reactor contents, the temperature measured at the exit ofthe reaction mixture from the reactor is the crucial temperature for thepurposes of the present invention.

If there is a significant temperature gradient between entry of thereaction mixture into the reactor and exit of the reaction mixture fromthe reactor, as may be the case in tubular reactors, the temperaturemeasured at the exit of the reaction mixture from the reactor is thecrucial temperature for the purposes of the present invention.

Preferably, the temperature in the reactors of the reactor cascade 3000increases from reactor 3000-1 to reactor 3000-i, i.e. the temperature inthe last reactor 3000-i is preferably higher than in the first reactor3000-1, and the temperature in two successive reactors between 3000-1and 3000-i may also be the same, but the temperature of each reactor3000-2, 3000-3, . . . , 3000-i is not lower than that of the precedingreactor. Preferably, however, the temperature in the reactors of thereactor cascade 3000 increases successively from reactor 3000-1 toreactor 3000-i, i.e. the temperature of each reactor 3000-2, 3000-3, . .. , 3000-i is higher than that of the preceding reactor.

It is likewise preferable to set

-   -   T₃₀₀₀₋₁ always to a value of 25.0° C. to 65.0° C., more        preferably 30.0° C. to 60.0° C., and    -   the temperature in each of the reactors downstream in flow        direction (3000-2, . . . , 3000-i) always to a value of 35.0° C.        to 200.0° C., more preferably of 50.0° C. to 180.0° C.

This makes it possible, in reactor 3000-1, to achieve a practicablecompromise between the aim of minimum by-product formation, especiallywith regard to N-methyl- and N-formyl-MDA, on the one hand (promoted bylower temperature) and the aim of ensuring a viscosity of the reactionmixture that assures practicable processibility of the product on theother hand (promoted by higher temperature).

All the aforementioned figures for preferred temperatures are applicablein all states of operation (A, T, E).

The respective values of n(1)/n(2) and n(7)/n(1) at the end of thetransition state (i.e. when m₂ has reached the target value for the endstate m₂(E)) are preferably retained for the duration of production withthe formaldehyde mass flow rate m₂(E).

Step (B-I), provided that the other requirements of the invention arecomplied with, can be conducted as known in principle from the priorart. The reaction mixture comprising the di- and polyamines of thediphenylmethane series is optionally neutralized with addition of waterand/or aniline. According to the prior art, the neutralization istypically effected at temperatures of, for example, 90.0° C. to 120.0°C. without addition of further substances. It can alternatively beeffected at a different temperature level in order, for example, toaccelerate the degradation of troublesome by-products. Suitable basesare, for example, the hydroxides of the alkali metal and alkaline earthmetal elements. Preference is given to employing aqueous NaOH. The baseused for neutralization is preferably used in amounts of greater than100%, more preferably 105% to 120%, of the amount stoichiometricallyrequired for the neutralization of the acidic catalyst used (see EP 1652 835 A1).

Step (B-II), provided that the other requirements of the invention arecomplied with, can be conducted as known in principle from the priorart. The neutralized reaction mixture comprising the di- and polyaminesof the diphenylmethane series is separated into an organic phasecomprising di- and polyamines of the diphenylmethane series and anaqueous phase. This can be assisted by the addition of aniline and/orwater. If the phase separation is assisted by addition of aniline and/orwater, they are preferably added already with vigorous mixing in theneutralization. The mixing can be effected here in mixing zones withstatic mixers, in stirred tanks or stirred tank cascades, or else in acombination of mixing zones and stirred tanks. The neutralized reactionmixture diluted by addition of aniline and/or water is then preferablysupplied to an apparatus which, owing to its configuration and/orinternals, is particularly suitable for separation into an organic phasecomprising MDA and an aqueous phase, preferably phase separation orextraction apparatuses according to the prior art, as described, forexample, in Mass-Transfer Operations, 3rd Edition, 1980, McGraw-HillBook Co, p. 477 to 541, or Ullmann's Encyclopedia of IndustrialChemistry (Vol. 21, Liquid-Liquid Extraction, E. Müller et al., pages272-274, 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, DOI:10.1002/14356007.b03_06.pub2) or in Kirk-Othmer Encyclopedia of ChemicalTechnology (see“http://onlinelibrary.wiley.com/book/10.1002/0471238961”, PublishedOnline: 15 Jun. 2007, pages 22-23) (mixer-settler cascades or settlingvessels).

If, in step (A-II), during the transition state, the flow rate of acidiccatalyst is increased significantly, the flow rate of base used in step(B-I) will of course also be increased correspondingly (and within thesame timeframe), in order that the requirement of the invention for thestoichiometric excess of base is always fulfilled, i.e. even in thetransition state. If, in step (A-II), during the transition state, theflow rate of acidic catalyst is lowered significantly, the flow rate ofbase used in step (B-I) will preferably also be lowered correspondingly(and likewise within the same timeframe), in order to avoid unnecessarysalt burdens.

Step (B-II) is preferably followed by further workup steps, namely:

Step (B-III): washing the organic phase (11) in a washing unit (6000,“washing vessel”) with washing liquid (13), followed by

Step (B-IV): separating the mixture (14) obtained in step (B-III) in aseparation unit (7000, “washing water separator”) into an organic phase(16) comprising di- and polyamines of the diphenylmethane series and anaqueous phase (15);

Step (B-V): distilling the organic phase (16) from step (B-IV) in adistillation apparatus (8000) to obtain the di- and polyamines of thediphenylmethane series (18), with removal of a stream (17) comprisingwater and aniline.

These steps can be conducted as known in principle from the prior art.In particular, it is preferable to conduct these steps continuously (asdefined above) as well.

It is particularly preferable that there is a subsequent washing (B-III)of the organic phase (11) with water (13) and a new separation of thewater phase (15) for removal of residual salt contents (preferably asdescribed in DE-A-2549890, page 3). After exiting from the phaseseparation in step (B-IV), the organic phase (16) comprising di- andpolyamines of the diphenylmethane series typically has a temperature of80.0° C. to 150.0° C.

Water and aniline are separated by distillation from the organic phasethus obtained, comprising di- and polyamines of the diphenylmethaneseries, as described in EP 1 813 597 B1. The organic phase preferablyhas a composition, based on the mass of the mixture, of 5.0% to 15% bymass of water and, according to the use ratios of aniline andformaldehyde, 5.0% to 90% by mass, preferably 5.0% to 40% by mass, ofaniline and 5.0% to 90% by mass, preferably 50% to 90% by mass, of di-and polyamines of the diphenylmethane series.

More preferably, the process of the invention also includes a furtherstep (C) in which recycling of stream (17) comprising water and aniline,optionally after workup, into step (A-I) and/or, if the optionaladdition of further aniline (1) in step (A-II) is conducted, into step(A-II) is undertaken.

In all embodiments of the invention, the setting of the operatingparameters to the target values for the end state is concluded with theend of the change in load (i.e. as soon as the mass flow rate of totalformaldehyde used m₂ has reached the target value for the end statem₂(E), time t₂). The values of n(1)/n(2) and n(7)/n(1) are therefore setstepwise or continuously, preferably continuously, to the target valuesfor the end state. In the case of stepwise setting of the respectiveparameter, the setting preferably comprises multiple stages, meaningthat, a target end value is established via one or more intermediatevalues, each of which is maintained for a particular period of time,between the starting value and the target end value. The term“continuous setting” also encompasses the case that the respectiveparameter is first varied in the “opposite direction”, as shown in FIG.1b -c.

In all embodiments of the invention, it is further preferable torestrict the change in load to a time not exceeding 120 minutes, i.e. tolimit the period within which the mass flow rate of total formaldehydeused m₂ is adjusted proceeding from m₂(A) to the target value for theend state m₂(E) (=duration of the transition state=period of time fromt₁ to t₂) is preferably to 120 minutes. The minimum duration of thetransition state is preferably 1.00 minute, more preferably 5.00 minutesand most preferably 30.00 minutes.

Detailed configurations of the invention are elucidated in detail in theappended examples.

The di- and polyamines of the diphenylmethane series that are obtainedin accordance with the invention can be reacted by the known methods,under inert conditions, with phosgene in an organic solvent to give thecorresponding di- and polyisocyanates of the diphenylmethane series, theMDI. The phosgenation can be conducted here by any of the methods knownfrom the prior art (e.g. DE-A-844896 or DE-A-19817691).

The procedure of the invention gives rise to the following advantagesfor the preparation of MDA:

-   i) The productivity of the MDA plant is higher because the    occurrence of off-spec material is minimized.-   ii) There is lower formation of by-products with acridine and    acridane structure and/or N-formyl- and N-methyl-MDA. Higher    proportions of these by-products can lead to quality problems in the    MDI conversion product.

Thus, the procedure of the invention, during a non-steady state (duringthe transition state), enables a technically seamless change in loadwithout subsequent outage periods in the steady state (the end state)that follows with constantly high quality of the desired MDA endproduct. The process of the invention also enables a rapid change inload and hence rapid reaction to events such as raw material shortage,etc.

EXAMPLES

The results outlined in the examples for the bicyclic content, theisomer composition and the content of N-methyl-4,4′-MDA are based oncalculations. The calculations are based partly on theoretical modelsand partly on process data collected in real operational experiments,the statistical evaluation of which created a mathematical correlationof running parameters and result (e.g. bicyclic content). The content ofN-formyl-4,4′-MDA is reported on the basis of operational experiencevalues. All percentages and ppm values reported are proportions by massbased on the total mass of the respective stream of matter. Theproportions by mass in the real operational experiments that gave thebasis for the theoretical model were ascertained by HPLC.

Reactor temperatures are based on the temperature of the respectiveprocess product at the exit from the reactor.

The MDA prepared, in all examples, has a residual aniline content in therange from 50 ppm to 100 ppm and a water content in the range from 200ppm to 300 ppm.

A. Reduction in Load Proceeding from Production with Nameplate Load

I. Starting State: Description of the Conditions Chosen for thePreparation of MDA at Nameplate Load

In a continuous reaction process, 23.20 t/h of feed aniline (containing90.0% by mass of aniline, 1) and 9.60 t/h of 32% aqueous formaldehydesolution (corresponding to a molar ratio of aniline (1):formaldehyde (2)of 2.25:1) are mixed and converted to the aminal (3) at a temperature of90.0° C. and a pressure of 1.40 bar (absolute) in a stirred reactiontank (1000). The reaction tank is provided with a cooler having acooling circuit pump. The reaction mixture leaving the reaction tank isguided into a phase separation apparatus (aminal separator, 2000) (step(A-I)).

After the phase separation to remove the aqueous phase (6), the organicphase (5) is admixed in a mixing nozzle with 30% aqueous hydrochloricacid (7) (protonation level 10%, i.e. 0.10 mol of HCl is added per moleof amino groups) and run into the first rearrangement reactor (3000-1).The first rearrangement reactor (called “vacuum tank”) is operated at50.0° C., which is ensured by means of evaporative cooling in a refluxcondenser at a pressure of 104 mbar (absolute). The reflux condenser ischarged with 0.50 t/h of fresh aniline. The rearrangement reaction isconducted to completion in a reactor cascade composed of a total ofseven reactors at 50.0° C. to 156.0° C. (i.e. 50.0° C. in reactor3000-1/60.0° C. in reactor 3000-2/83.0° C. in reactor 3000-3/104.0° C.in reactor 3000-4/119.0° C. in reactor 3000-5/148.0° C. in reactor3000-6/156.0° C. in reactor 3000-7) (step (A-II)).

On completion of reaction, the reaction mixture (8-i) obtained isadmixed with 32% sodium hydroxide solution in a molar ratio of 1.10:1sodium hydroxide to HCl and reacted in a stirred neutralization vessel(4000) (step (B-I)). The temperature here is 115.0° C. The absolutepressure is 1.40 bar. The neutralized reaction mixture (10) is thenseparated in a neutralization separator (5000) into an aqueous lowerphase (12), which is guided to a wastewater collection vessel, and intoan organic phase (11) (step (B-II)).

The organic upper phase (11) is guided to the washing and washed withcondensate (13) in a stirred washing vessel (6000) (step (B-III)). Afterthe washing water (15) has been separated from the biphasic mixture (14)obtained in the washing vessel (6000) in a washing water separator(7000, step (B-IV)), the crude MDA (16) thus obtained is freed of waterand aniline (removed together as stream 17) by distillation, and 17.00t/h of MDA (18) were obtained as bottom product (step (B-V)).

MDA prepared in this way has an average composition of 45.2% 4,4′-MDA,5.5% 2,4′-MDA, 0.3% 2,2′-MDA, i.e. a total bicyclic content of 51.0% anda proportion by mass of 2,4′-MDA of 10.8%, and also 0.3%N-methyl-4,4′-MDA and 0.3% N-formyl-4,4′-MDA, the remainder to 100%consisting essentially of higher homologs (PMDA) and isomers thereof.

II. Target End State: Production at Half-Load

Example 1 (Comparative Example): Reduction in Load of the MDA Plant fromNameplate Load to Half-Load (=50% of Nameplate Load), where then(1)/n(2) Ratio and the n(7)/n(1) Ratio are Kept the Same, the Aniline,Formalin, Hydrochloric Acid and Sodium Hydroxide Solution Feedstocks areReduced Simultaneously and the Temperatures in the Vacuum Tank (3000-1),the Downstream Reactors and the Exit Temperature of the Crude MDASolution in the Last Rearrangement Reactor (3000-7) Remain the Same

The MDA plant, as described above under A.I, is operated at a productioncapacity of 17.0 t/h of MDA. Owing to lower product demand theproduction load is to be halved. For this purpose, at the same time, thefeed rates of aniline and formalin to the aminal reactor (1000) areadjusted to the new production load within 120 minutes. The formalinrate is reduced to 4.80 t/h. The aniline feed rate is reduced to 11.35t/h. At the same time, the flow rate of hydrochloric acid into themixing nozzle in the feed to the first rearrangement reactor (3000-1) ishalved. The first rearrangement reactor is still operated at 50.0° C.,which is ensured by means of the evaporative cooling in the refluxcondenser at 104 mbar (absolute). The reflux condenser is still chargedwith 0.50 t/h of fresh aniline. The rearrangement reaction is conductedto completion in a reactor cascade at 50.0° C. to 156.0° C. (50.0° C. inreactor 3000-1/60.0° C. in reactor 3000-2/83.0° C. in reactor3000-3/104.0° C. in reactor 3000-4/119.0° C. in reactor 3000-5/148.0° C.in reactor 3000-6/156.0° C. in reactor 3000-7). On completion ofreaction, the reaction mixture obtained, as described in the generalconditions for preparation of MDA, is neutralized with sodium hydroxidesolution, with reduction of the amount of sodium hydroxide solutionwithin the same time window as formalin and aniline, and then worked upto give MDA (18).

40 hours after commencement of the change in load, the MDA in the feedto the MDA tank has a composition of 44.7% 4,4′-MDA, 5.2% 2,4′-MDA, 0.2%2,2′-MDA, i.e. a total bicyclic content of 50.1% (determined by HPLC)and also 0.3% N-methyl-4,4′-MDA and 0.3% N-formyl-4,4′-MDA, theremainder to 100% consisting essentially of higher homologs (PMDA) andisomers thereof. The MDA has an elevated proportion of unwantedby-products with acridine and acridane structure. In this regard, seealso section C further down.

Example 2 (Inventive): Reduction in Load of the MDA Plant from NameplateLoad to Half-Load, where the n(1)/n(2) Ratio is Increased, the Anilineand Formalin Feedstocks are Reduced Simultaneously and where then(7)/n(1) Ratio is Reduced, Hydrochloric Acid and Sodium HydroxideSolution are Reduced Simultaneously, the Temperatures in the Vacuum Tank(3000-1), the Downstream Reactors and the Exit Temperature of the CrudeMDA Solution in the Last Rearrangement Reactor (3000-7) Remain the Same

The MDA plant, as described above under A.I, is operated at a productioncapacity of 17.0 t/h of MDA. Owing to lower product demand theproduction load is to be halved. For this purpose, at the same time, thefeed rates of aniline and formalin to the aminal reactor (1000) areadjusted to the new production load within 120 minutes. The formalinrate is reduced to 4.80 t/h. The aniline feed rate is reduced to 11.90t/h. The flow rate of hydrochloric acid into the mixing nozzle in thefeed to the first rearrangement reactor is adjusted within the sameperiod as aniline and formalin while reducing the protonation level to7.0%. The first rearrangement reactor (3000-1) is still operated at50.0° C., which is ensured by means of the evaporative cooling in thereflux condenser at 104 mbar (absolute). The reflux condenser is stillcharged with 0.50 t/h of fresh aniline. The rearrangement reaction isconducted to completion in a reactor cascade at 50.0° C. to 156° C.(50.0° C. in reactor 3000-1/60.0° C. in reactor 3000-2/83.0° C. inreactor 3000-3/104.0° C. in reactor 3000-4/119.0° C. in reactor3000-5/148.0° C. in reactor 3000-6/156.0° C. in reactor 3000-7) (step(A-II)). On completion of reaction, the reaction mixture obtained, asdescribed in the general conditions for preparation of MDA, isneutralized with sodium hydroxide solution, with reduction of the amountof sodium hydroxide solution within the same time window as formalin andaniline and HCl with retention of the molar ratio of 1.10:1 sodiumhydroxide solution to HCl, and then worked up to give the desired MDAtype, obtaining 8.5 t/h of MDA (18) as the bottom product from thedistillation.

40 hours after commencement of the change in load, the MDA in the feedto the MDA tank has a composition of 45.9% 4,4′-MDA, 5.6% 2,4′-MDA, 0.3%2,2′-MDA, i.e. a total bicyclic content of 51.8% and also 0.2%N-methyl-4,4′-MDA and 0.3% N-formyl-4,4′-MDA, the remainder to 100%consisting essentially of higher homologs (PMDA) and isomers thereof.The product differs only insignificantly from the MDA stream (18) whichis obtained on average in the starting state as described in A.I. Anelevated proportion of unwanted by-products with acridine and acridanestructure is not formed.

Table 1 below compares the results from section A.

TABLE 1 Comparison of the examples from section A End state End stateHalf-load Half-load Starting state Example 1 Example 2 Nameplate load(comp.) (inv.) Aniline (90%) in reactor 1000 [t/h] 23.20 11.35 11.90Aniline in reactor 3000-1 [t/h] 0.50 0.50 0.50 Formalin (32%) in reactor1000 [t/h] 9.60 4.80 4.80 n(1)/n(2) 2.25 2.25 2.35 [n(1)/n(2)(T)(t =t₂)]/[n(1)/n(2)(A)] — 1.00 1.04 Protonation level [%] 10 10 7.0n(7)/n(1) 0.10 0.10 0.070 [n(7)/n(1)(T)(t = t₂)]/[n(7)/n(1)(A)] — 1.000.70 Temp. gradient in reactor cascade 50.0 → 156.0 50.0 → 156.0 50.0 →156.0 3000 [° C.] T(3000-1) [° C.] 50.0 50.0 50.0 T(3000-2) [° C.] 60.060.0 60.0 T(3000-3) [° C.] 83.0 83.0 83.0 T(3000-4) [° C.] 104.0 104.0104.0 T(3000-5) [° C.] 119.0 119.0 119.0 T(3000-6) [° C.] 148.0 148.0148.0 T(3000-7) [° C.] 156.0 156.0 156.0 Production capacity [t/h] 17.008.50 8.50 4,4′-MDA [%] 45.2 44.7 45.9 2,4′-MDA [%] 5.5 5.2 5.6 2,2′-MDA[%] 0.3 0.2 0.3 N-methyl-MDA [%] 0.3 0.3 0.2 N-formyl-MDA [%] 0.3 0.30.3 Bicyclic content [%] 51.0 50.1 51.8 Comment — Elevated Product hasproportion of comparable unwanted by- isomer products with compositionand a acridine and similar by-product acridane structure spectrum to theproduct in the starting stateB. Increase in Load Proceeding from Production at Half-LoadI. Starting State: Description of the Conditions Chosen for thePreparation of MDA at Half-Load

(N.B. There are of course various options for operating a productionplant 10 000 at half-load.) The conditions set in example 2 are oneoption; for examples 3 and 4 which follow, another option was chosen forthe “half-load” starting state.)

The reaction is operated as described above under A.I for nameplate loadwith the following differences:

11.35 t/h of feed aniline (containing 90.0% by mass of aniline);

4.80 t/h of 32% aqueous formaldehyde solution (i.e. the molar ratio ofaniline:formaldehyde is 2.25:1);

50.0° C. in reactor 3000-1/60.0° C. in reactor 3000-2/81.0° C. inreactor 3000-3/95.0° C. in reactor 3000-4/116.0° C. in reactor3000-5/144.0° C. in reactor 3000-6/146.0° C. in reactor 3000-7;

Bottom product of 8.50 t/h of MDA (18).

MDA prepared in this way has an average composition of 46.3% 4,4′-MDA,5.0% 2,4′-MDA, 0.2% 2,2′-MDA, i.e. a total bicyclic content of 51.5% andalso 0.3% N-methyl-4,4′-MDA and 0.3% N-formyl-4,4′-MDA, the remainder to100% consisting essentially of higher homologs (PMDA) and isomersthereof.

II. Target End State: Production at Nameplate Load

Example 3 (Comparative Example): Increase in Load of the MDA Plant fromHalf-Load to Nameplate Load, where the n(1)/n(2) Ratio and the n(7)/n(1)Ratio are Kept the Same, the Aniline, Formalin, Hydrochloric Acid andSodium Hydroxide Solution Feedstocks are Increased Simultaneously andthe Temperatures in the Vacuum Tank (3000-1), the Downstream Reactorsand the Exit Temperature of the Crude MDA Solution in the LastRearrangement Reactor (3000-7) Remain the Same

The MDA plant, as described above under B.I, is operated at a productioncapacity of 8.50 t/h of MDA. Owing to higher product demand theproduction load is to be doubled to nameplate load. For this purpose, atthe same time, the feed rates of aniline and formalin to the aminalreactor are adjusted to the new production load within 120 minutes. Theformalin rate was increased to 9.60 t/h. The aniline feed rate wasincreased to 23.20 t/h. The flow rate of hydrochloric acid into themixing nozzle in the feed to the first rearrangement reactor isincreased within the same period as aniline and formalin with retentionof the protonation level of 10% (i.e. with retention of the n(7)/n(1)ratio). The first rearrangement reactor (3000-1) is still operated at50.0° C., which is ensured by means of the evaporative cooling in thereflux condenser at 104 mbar (absolute). The reflux condenser is chargedwith 0.50 t/h of fresh aniline. The rearrangement reaction is stillconducted to completion in the reactor cascade at 50.0° C. to 145° C.(50.0° C. in reactor 3000-1/60.0° C. in reactor 3000-2/82.0° C. inreactor 3000-3/96.0° C. in reactor 3000-4/117.0° C. in reactor3000-5/142.0° C. in reactor 3000-6/145.0° C. in reactor 3000-7).

After the reaction, the reaction mixture obtained is admixed with 32%sodium hydroxide solution in a molar ratio of 1.10:1 sodium hydroxidesolution to HCl and reacted in a stirred neutralization vessel,increasing the amount of sodium hydroxide solution within the same timewindow as formalin, aniline and HCl with retention of the molar ratios.The further workup is effected as described above under A.I. At the endof the transition state, 17.0 t/h of a bottom product are obtained.

Result: “MDA” thus prepared has not been completely rearranged and stillcontains partly rearranged products such as aminobenzylanilines, whichlead to quality problems in the subsequent phosgenation to give MDI, forexample distinctly elevated color values in the resulting MDI product.The MDA thus prepared, and also the resulting MDI, are off-spec product.

Example 4 (Inventive)

Increase in Load of the MDA Plant from Half-Load to Nameplate Load,where the n(1)/n(2) Ratio is Lowered, the Aniline and FormalinFeedstocks are Increased Simultaneously, and where the n(7)/n(1) Ratiois Increased, Hydrochloric Acid and Sodium Hydroxide Solution areIncreased Simultaneously, the Temperatures in the Vacuum Tank (3000-1),the Downstream Reactors and the Exit Temperature of the Crude MDASolution in the Last Rearrangement Reactor (3000-7) Remain the Same

The MDA plant, as described above under B.I, is operated at a productioncapacity of 8.50 t/h of MDA. Owing to higher product demand theproduction load is to be doubled to nameplate load. For this purpose, atthe same time, the feed rates of aniline and formalin to the aminalreactor are adjusted to the new production load within 120 minutes. Theformalin rate is increased to 10.00 t/h. The aniline feed rate isincreased to 23.20 t/h. The flow rate of hydrochloric acid into themixing nozzle in the feed to the first rearrangement reactor is adjustedwithin the same period as aniline and formalin with an increase of theprotonation level to 13%. The first rearrangement reactor (3000-1) isstill operated at 50.0° C., which is ensured by means of the evaporativecooling in the reflux condenser at 104 mbar (absolute). The refluxcondenser is charged with 0.50 t/h of fresh aniline. The rearrangementreaction is conducted to completion in the reactor cascade at 50.0° C.to 146° C. (50.0° C. in reactor 3000-1/60.0° C. in reactor 3000-2/81.0°C. in reactor 3000-3/95.0° C. in reactor 3000-4/117.0° C. in reactor3000-5/144.0° C. in reactor 3000-6/146.0° C. in reactor 3000-7) (stepA-II)).

After the reaction, the reaction mixture obtained is admixed with 32%sodium hydroxide solution in a molar ratio of 1.10:1 sodium hydroxidesolution to HCl and reacted in a stirred neutralization vessel,increasing the amount of sodium hydroxide solution within the same timewindow as formalin, aniline and hydrochloric acid. The temperature is115.0° C. The absolute pressure is 1.40 bar. The further workup iseffected as described further up under A.I. At the end of the transitionstate, the bottom product obtained is 17.00 t/h of MDA (18).

20 hours after commencement of the change in load, the MDA in the feedto the MDA tank has a composition of 45.8% 4,4′-MDA, 4.9% 2,4′-MDA, 0.2%2,2′-MDA, i.e. a total bicyclic content of 50.9% and also 0.3%N-methyl-4,4′-MDA and 0.3% N-formyl-4,4′-MDA, the remainder to 100%consisting essentially of higher homologs (PMDA) and isomers thereof.The product differs only insignificantly from the MDA stream (18) whichis obtained on average in the starting state as described in B.I. Anelevated proportion of unwanted by-products with acridine and acridanestructure is not formed.

Table 2 below compares the results from section B.

TABLE 2 Comparison of the examples from section B End state End stateStarting state Nameplate load Nameplate load Half-load Example 3 (comp.)Example 4 (inv.) Aniline (90%) in reactor 1000 [t/h] 11.35 23.20 23.20Aniline in reactor 3000-1 [t/h] 0.50 0.50 0.50 Formalin (32%) in reactor1000 [t/h] 4.80 9.60 10.00 n(1)/n(2) 2.25 2.25 2.16 [n(1)/n(2)(T)(t =t₂)]/[n(1)/n(2)(A)] — 1.00 0.96 Protonation level [%] 10 10 13 n(7)/n(1)0.10 0.10 0.13 [n(7)/n(1)(T)(t = t₂)]/[n(7)/n(1)(A)] — 1.00 1.30 Temp.gradient in reactor cascade 50.0 → 146.0 50.0 → 145.0 50.0 → 146.0 3000[° C.] T(3000-1) [° C.] 50.0 50.0 50.0 T(3000-2) [° C.] 60.0 60.0 60.0T(3000-3) [° C.] 81.0 82.0 81.0 T(3000-4) [° C.] 95.0 96.0 95.0T(3000-5) [° C.] 116.0 117.0 117.0 T(3000-6) [° C.] 144.0 142.0 144.0T(3000-7) [° C.] 146.0 145.0 146.0 Production capacity [t/h] 8.50 17.0017.00 4,4′-MDA [%] 46.3 — 45.8 2,4′-MDA [%] 5.0 — 4.9 2,2′-MDA [%] 0.2 —0.2 N-methyl-MDA [%] 0.3 — 0.3 N-formyl-MDA [%] 0.3 — 0.3 Bicycliccontent [%] 51.5 — 50.9 Comment — MDA has been Product has incompletelycomparable rearranged and still isomer contains partly composition and arearranged similar by-product products such as spectrum to theaminobenzylanilines product in the which lead to starting state qualityproblems in the subsequent phosgenation to give MDI, for exampledistinctly elevated color valuesC. Fundamental Experiments for Formation of by-Products with Acridineand Acridane Structure

In a series of experiments, the amount of aqueous 30% hydrochloric acidrequired in each case to achieve the desired protonation level (seetable 3 below) was added to a 2,2′-MDA solution in aniline preheated to100.0° C. The 2,2′-MDA concentration in each of the individualexperiments was 1.0% by mass; in addition, the solutions containedoctadecane as an internal standard for gas chromatography (GC) analysis.The resulting mixture was transferred as quickly as possible by means ofa peristaltic pump to a Mai glass autoclave preheated to 120.0° C. andheated to the reaction temperature envisaged (see table 3). Onattainment of the desired reaction temperature, the first sample wastaken (time=zero). In order to monitor the progress of the reaction,further samples were taken after 30, 60, 120 and 240 minutes andanalyzed by means of GC analysis. Reaction conditions and experimentalresults are collated in table 3 below.

Scheme 1: Formation of acridane and acridine from 2,2′-MDA

TABLE 3 Laboratory experiments for formation of the acridane andacridine secondary components Sum total Reaction Protonation (acridine +temperature level Time acridane) Experiment [° C.] [%] [min] [ppm] 1160° C. 10% 15 79 30 208 60 421 120 925 240 1775 2 170° C. 25% 0 977 302554 60 3950 120 5647 240 8894 3 160° C. 25% 0 364 30 930 60 1594 1202681 240 4354 4 180 10% 0 606 30 1630 60 2613 120 3932 240 5294 5 17010% 0 292 30 794 60 1533 120 2406 240 3932 6 160  5% 0 0 30 0 60 0 120 0240 0 7 170  5% 0 0 30 0 60 0 120 0 240 218 8 180  5% 0 0 30 402 60 834120 1355 240 2380

It is found that, with rising temperature and rising hydrochloric acidconcentration, the formation of the acridine and acridane secondarycomponents from 2,2′-MDA occurs to an increased degree.

The invention claimed is:
 1. A process for preparing di- and polyaminesof the diphenylmethane series from aniline (1) and formaldehyde (2) in aproduction plant (10 000), where the molar ratio of total aniline used(1) to total formaldehyde used (2), n(1)/n(2), is always greater than1.6, comprising: (A-I) reacting aniline (1) and formaldehyde (2) in theabsence of an acidic catalyst to obtain a reaction mixture (4)comprising an aminal (3), and then at least partly separating an aqueousphase (6) from the reaction mixture (4) to obtain an organic phase (5)comprising the aminal (3); (A-II) contacting the organic phase (5) whichcomprises the aminal obtained in step (A-I) with an acidic catalyst (7)in a reactor cascade (3000) composed of i reactors connected in series(3000-1, 3000-2, . . . , 3000-i), where i is a natural number from 2 to10, wherein the first reactor (3000-1) in flow direction is operated ata temperature T3000-1 in the range from 25.0° C. to 65.0° C. and ischarged with stream (5) and acidic catalyst (7) and optionally withfurther aniline (1) and/or further formaldehyde (2), and every reactordownstream in flow direction (3000-2, . . . , 3000-i) is operated at atemperature of more than 2.0° C. above T3000-1 and is charged with thereaction mixture obtained in the reactor immediately upstream; (B)isolating the di- and polyamines of the diphenylmethane series from thereaction mixture (8-i) obtained from step (A-II) in the last reactor(3000-i) by a process comprising: (B-I) adding a stoichiometric excessof base (9), based on the total amount of acidic catalyst used (7), tothe reaction mixture (8-i) obtained in the last reactor (3000-i) in step(A-II) to obtain a reaction mixture (10); and (B-II) separating thereaction mixture (10) obtained in step (B-I) into an organic phase (11)comprising di- and polyamines of the diphenylmethane series and anaqueous phase (12); wherein in the event of a change in the productioncapacity from a starting state A with a mass flow rate in the startingstate of total aniline used of m₁(A)≠0, a mass flow rate in the startingstate of total formaldehyde used of m₂(A)=X(A)·m₂(N), where X(A) is adimensionless number>0 and ≤1 and m₂(N) denotes the nameplate load ofthe production plant (10 000), a molar ratio in the starting state oftotal aniline used (1) to total formaldehyde used (2) of n(1)/n(2)(A)and a molar ratio in the starting state of total acidic catalyst used tototal aniline used of n(7)/n(1)(A) to an end state E with a mass flowrate in the end state of total aniline used of m₁(E)≠0, a mass flow ratein the end state of total formaldehyde used of m₂(E)=X(E)·m₂(N), whereX(E) is a dimensionless number>0 and ≤1, a molar ratio in the end stateof total aniline used (1) to total formaldehyde used (2) of n(1)/n(2)(E)and a molar ratio in the end state of total acidic catalyst used tototal aniline used of n(7)/n(1)(E); by a quantity ΔX=|X(E)−X(A)|, withΔX≥0.10, wherein the process comprises at least one change in productioncapacity that commences at a time t₁ and concludes at a time t₂, wherethe temperature of each reactor j of the reactor cascade (3000),T_(3000-j) is: (T_(3000-j)(A)−2.0° C.)≤T_(3000-j)(E)≤(T_(3000-j)(A)+2.0°C.) wherein, in the period from t₁ to t₂, the transition state T, with amolar ratio of total aniline used (1) to total formaldehyde used (2) ofn(1)/n(2)(T) and a molar ratio of total acidic catalyst used to totalaniline used of n(7)/n(1)(T), (i) the temperature in the first reactor(3000-1) in flow direction from step (A-II) is adjusted to a value thatdiffers from the temperature in that reactor during the starting state Aby not more than 10.0° C.; (ii) the temperature in all reactorsdownstream in flow direction (3000-2, . . . , 3000-i), by comparisonwith the starting state A, is kept the same in each case within a rangeof variation of ±2.0° C.; (iii-1) in the case that m₂(E)>m₂(A),n(1)/n(2)(T) and n(7)/n(1)(T) are adjusted such that, at time t₂:0.80·n(1)/n(2)(A)≤n(1)/n(2)(T)≤0.99·n(1)/n(2)(A); and1.01·n(7)/n(1)(A)≤n(7)/n(1)(T)≤2.00·n(7)/n(1)(A); where it is always thecase throughout the transition state prior to attainment of time t2that: 0.80·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.20·n(1)/n(2)(A); and0.83·n(7)/n(1)(A)≤n(7)/n(1)(T)≤2.00·n(7)/n(1)(A); (iii-2) in the casethat m₂(E)≤m₂(A), n(1)/n(2)(T) and n(7)/n(1)(T) are adjusted such that,at time t₂: 1.01·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.50·n(1)/n(2)(A); and0.50·n(7)/n(1)(A)≤n(7)/n(1)(T)≤0.99·n(7)/n(1)(A); where it is always thecase throughout the transition state prior to attainment of time t2that: 0.90·n(1)/n(2)(A)≤n(1)/n(2)(T)≤1.50·n(1)/n(2)(A); and0.50·n(7)/n(1)(A)≤n(7)/n(1)(T)≤1.11·n(7)/n(1)(A).
 2. The process ofclaim 1, in which the temperature in the reactors of the reactor cascade3000 increases from reactor 3000-1 to reactor 3000-i in all states ofoperation (A, T, E).
 3. The process of claim 1, in which it is alwaysthe case that T3000-1 is set to a value in the range from 25.0° C. to65.0° C. and the temperature in each of the reactors downstream in flowdirection (3000-2, . . . , 3000-i) is set to a value in the range from35.0° C. to 200.0° C.
 4. The process of claim 3, in which it is alwaysthe case that T3000-1 is set to a value in the range from 30.0° C. to60.0° C. and the temperature in each of the reactors downstream in flowdirection (3000-2, . . . , 3000-i) is set to a value in the range from50.0° C. to 180.0° C.
 5. The process of claim 1, in which the acidiccatalyst (7) is a mineral acid.
 6. The process of claim 1, in which step(B) further comprises: (B-III) washing the organic phase (11) withwashing liquid (13); (B-IV) separating the mixture (14) obtained in step(B-III) into an organic phase (16) comprising di- and polyamines of thediphenylmethane series and an aqueous phase (15); and (B-V) distillingthe organic phase (16) from step (B-IV) to obtain the di- and polyaminesof the diphenylmethane series (18), with removal of a stream (17)comprising water and aniline.
 7. The process of claim 6, additionallycomprising: (C) recycling stream (17), optionally after workup, intostep (A-I) and/or, if the optional addition of further aniline (1) instep (A-II) is conducted, into step (A-II).
 8. The process of claim 1,in which the molar ratio of total aniline used (1) to total formaldehydeused (2), n(1)/n(2), in all states of operation (A, T, E) is adjusted toa value of 1.6 to
 20. 9. The process of claim 1, in which the targetvalues of n(1)/n(2) and n(7)/n(1) for the end state E are establishedcontinuously in the transition state.
 10. The process of claim 1, inwhich the values for n(1)/n(2) and n(7)/n(1) that exist in each case attime t₂ are retained for the duration of the production with theformaldehyde mass flow rate m₂(E).
 11. The process of claim 1, in whichthe period from t₁ to t₂ lasts from 1.00 minute to 120 minutes.